<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>injectCSS</title>
</head>
<body>
    <script>
        const injectCSS = css => {
            const el = document.createElement('style');
            el.innerText = css;
            el.type = 'text/css';
            document.head.appendChild(el);
            return el;
        }
        injectCSS(`body,html{ background:linear-gradient(135deg,#112233 10%,#222334 90%);height:100%; }`);
    </script>
</body>
</html>